Licenses

Linux is largely a community of volunteers and as such represents one of the largest altruistic efforts on earth. This includes companies who decide to contribute their own software into the public domain for free use. The continued success of sharing depends on licenses that keep software free and usable for anyone who wants to use it. However, there must be a method for Linux users and developers to make money, as well. Licensing helps protect each of these efforts. See the Wikipedia Free Software Licensing article and the GNU operating system licensing page for more complete information.

GPL license

The GPLv3 license (and the Affero GPLv3 license for network-based software) intends that the software module or package is free to use in any environment, and furthermore, any software that relies on that GPLv3-licensed module must in turn also be completely free. Commercial and proprietary software packages can't use or incorporate GPLv3-licensed modules.

LGPL license

The Lesser GPL license intends that the software module or package is free to use in any environment, including in commercial and proprietary software packages. This allows companies to develop proprietary packages which includes LGPL-licensed modules, from which they can make a profit. The disadvantage is that their products (which benefit from the LGPL-licensed modules) are not required to be in the public domain in turn. (Many companies often later donate their entire package into the public domain, however, after they no longer make a profit from them.)

Apache license

The Apache license has been around a long time. It is compatible with the GPLv3 license, but, unlike the GPLv3 license, it does not require modified software to retain the Apache license. In other words, Apache-licensed software can be modified and the modified software then made proprietary (and therefore not returned to the open source community).

BSD license

The BSD license is similar to a public domain license. There are currently many confusing iterations of the BSD license, mostly regarding attribution notices and advertising that is required to be provided along with any software derivatives. The BSD license allows the option of propagation of either (otherwise-licensed) free open source restrictions or proprietary restrictions. It therefore allows a mix of (otherwise-licensed) proprietary modules and open sourced-licensed modules to co-exist in the same package. This flexibility has made the BSD license popular with complex distributions (such as the (BSD Unix-based) Mac OS X operating system, for example).

Proprietary licenses

There is a vast array of proprietary licenses, all different. You never know what your limitations for software are unless you read every word. Most are attempts by lawyers to have an opportunity to create a lawsuit in the future. Some may be called "free" licenses but have many limitations which you will not be aware of until you are in the middle of a lawsuit. No license outside of the GPLv3 license is recommended. Be careful when committing your organization to a mission-critical software package with a proprietary license. Also see this outstanding article on the Open Source Enterprise Trap.